Drawbacks of Old Quantum Theory

Planck's quantum hypothesis with its application and extension to explain the black body radiation like the photo-electric effect, the Compton effect, the variation of specific heat of solid with temperature and the spectrum of hydrogen is now called the Old quantum theory. Through these phenomena are successfully explained by the theory, there are numerous drawbacks of the theory. A few of them are as follows.

1.) Bohr's quantization rules are arbitrary. The theory does not provide a physical explanation for the assumptions.

2.) The old quantum theory cannot be applied to explain the spectra of helium and of more complex atoms.

3.) It can provide only a qualitative and incomplete explanation of the intensities of the spectral lines.

4.) It can not explain the dispersion of light.

5.) The theory of non-harmonic vibrations of systems cannot be applied to explain the vibrations of systems.